Rework rustflags-related stuff by using rustc-wrapper

[taiki-e]

Based on several past discussions, I suspect this might be more optimal than the current approach in terms of performance, disk usage, and interaction with rustflags that users configure in various ways.

#387 (comment)

[jschwe]

I did a quick test in servo, and using the simple python script below as a RUSTC_WORKSPACE_WRAPPER reduced the profile size to around 1/4 of the previous value. I did not test performance explicitly, but it felt like the tests also completed faster.
How hard would it be to integrate this into cargo llvm-cov, so I don't need to separate build and running?

#!/usr/bin/env python3

"""
This is a simple script intended to be used as a `RUSTC_WORKSPACE_WRAPPER`, adding the
required flags for code coverage instrumentation, only to the local libraries in our
workspace. This reduces the runtime overhead and the profile size significantly.
We are not interested in the code coverage metrics of outside dependencies anyway.
"""

import os
import sys

def main():
    # The first argument is the path to rustc, followed by its arguments
    args = sys.argv[1:]
    args += ["-Cinstrument-coverage=true", "--cfg=coverage"]

    # Execute rustc with the modified arguments
    os.execvp(args[0], args)


if __name__ == "__main__":
    main()

[taiki-e]

Implemented in 0.7.0 (#471).